Fuel cell assemblies employing a plurality of individual fuel cell modules are well known. Each module has an anode and a cathode. In a proton-exchange fuel cell, the anode and cathode are separated by a catalytic proton exchange membrane (PEM) in which the modules in the stack typically are connected in series electrically through bipolar plates to provide a desired total output voltage. Fuel in the form of hydrogen and water vapor, or hydrogen-containing mixtures such as “reformed” hydrocarbons, is flowed through a first set of reaction channels formed in a first surface of the bipolar plate adjacent the anode. Oxygen, typically in the form of air, is flowed through a second set of reaction channels formed in a second surface of the bipolar plate adjacent the cathode.
In a PEM fuel cell, hydrogen is catalytically oxidized at the anode-membrane interface. The resulting proton, H+, migrates through the membrane to the cathode-membrane interface where it combines with ionic oxygen to form water. Electrons flow from the anode through a load to the cathode, doing electrical work in the load.
In fuel cells, a long-term electrical continuity problem is well known in the art. Metals typically used to form bipolar plates, for example, aluminum a stainless steel, either corrode or form high-resistance oxide passivation layers on the surface of the bipolar plates because of electrochemical activity at these surfaces. These high resistant oxide layers limit the current-collecting ability of the bipolar plates, significantly lower the efficiency and output of a fuel cell. In the prior art, bipolar plates are known to be coated with noble metals such as gold and platinum to prevent corrosion and the formation of high resistant passivation layers on the electrical contact surfaces, but such coatings are so expensive as to impact the widespread use of cost-effective fuel cells.
What is needed is a simple and cost-effective means for maintaining high electrical conductivity of the electrical-contact surfaces of a bipolar plate.
It is a principal object of the present invention to provide an improved bipolar plate which is simple and inexpensive to manufacture and which maintains high electrical conductivity of the surface during use in a fuel cell.
It is a further object of the invention to increase the durability and reliability of a fuel cell.